Sales order qnty and po qty update

Dear SD Experts

This Question is Related To sales order To PO

Recently in one post i have Asked but that is not Good to interfere in that post , that is the reason i am asking again

I have created one sales order with 100 qnty --And PO --100 Qty

Now i have changed Po qty 150 and i have checked in Sales order here also i am able to find 150 qnty ...Both places sales order qty level --and s.lines level

And i have changed in sales order qty but it is not updated in PO

What  i am thinking is This is Standard one ..so could you anybody tell me if it is wrong beaver of system please tell me where is the route cause

Thanks a lot